CN103338023A - Filter eliminating external noises irrelevant to partial discharge during high-frequency partial discharge measurement - Google Patents

Abstract

The invention discloses a filter for eliminating external noise irrelevant to partial discharge during high-frequency partial discharge measurement, comprising: an input unit, which has a signal input terminal; an integral filter unit, which is connected to the input unit; and not A balance-balance conversion unit, which is connected to the integral filter unit. The filter of the present invention has a structure in which the integral filter unit is connected to the unbalanced-balanced conversion unit, and has the advantages of effectively suppressing interference, extracting partial discharge pulse signals, and avoiding interference between nonlinear loads and linear loads. Affected by the failure of the filter's external power supply system and other uncertain factors; it can be used in conjunction with other passive filters and active filters; when the frequency of interference noise such as carrier communication on site changes, there is no need to change the parameters of the filter , the entire filter has a simple structure, safe and reliable operation, and flexible installation positions.

Description

Filter for eliminating external noise not related to PD in high frequency PD measurement

technical field

The invention belongs to the technical field of power electronics, and in particular relates to a filter for eliminating external noise irrelevant to partial discharge during high-frequency partial discharge measurement.

Background technique

Partial discharge refers to the phenomenon of partial repeated breakdown and extinguishment under a certain applied voltage due to the weakness in the internal insulation of electrical equipment. Usually the electric field intensity in the discharge area is relatively large, but the discharge energy is very small, so the existence of partial discharge does not affect the short-term insulation strength of electrical equipment. When partial discharge occurs inside the medium, it is accompanied by many phenomena, including electric ones, such as the generation of electric pulses, the increase of dielectric loss and electromagnetic wave radiation; there are also non-electrical ones, such as light, heat, noise, and medium substances. Changes in stress and chemistry, etc.

During the measurement of partial discharge, due to the harsh environmental factors of the equipment site, many interference noises will be introduced in the on-site monitoring. These interference noises mainly include: continuous periodic interference and pulse interference, etc. Among them, periodic interference mainly includes carrier communication, The harmonics and pulse interference of radio communication and system are divided into periodic pulse interference and random pulse interference, while the periodic pulse interference is mainly the pulse interference in the ground network, and the random pulse interference is mainly external discharge. The amplitude of the above-mentioned interference noise may be many times that of the PD signal, and the PD signal and the interference noise often overlap in the frequency domain, so noise reduction technology is the key to PD online monitoring.

The Chinese patent with the notification number CN2326965 discloses a filter for on-line monitoring of generator fault discharge. The filter includes a program-controlled combined band-pass analog filter. The output of the filter is connected to the input of a high-speed data collector. , the digital signal output by the high-speed data collector is input into the software adaptive digital filter in the computer for noise elimination. The filter can suppress intermediate frequency radio interference, power system carrier communication interference and random pulse interference on site. But the disadvantage is that the filter is very slow in eliminating the sensitive characteristics of the interference noise, and it is not flexible in the control application, so that the performance of the monitoring system cannot meet the practical requirements.

Contents of the invention

In view of this, the object of the present invention is to provide a filter for eliminating external noise irrelevant to partial discharge during high-frequency partial discharge measurement, which has good sensitivity in eliminating interference noise and is flexible in control application.

In order to achieve the above object, the technical solution of the present invention is achieved as follows: a filter for eliminating external noise irrelevant to partial discharge during high-frequency partial discharge measurement, characterized in that it includes: an input unit, which has a signal input terminal ; an integral filter unit, which is connected to the input unit; and an unbalanced-balanced conversion unit, which is connected to the integral filter unit.

The input unit includes an electronic switch.

The integral filter unit includes an n-th filter circuit connected to an n+1-th filter circuit, where n is an integer greater than or equal to 1.

The nth filtering circuit is connected with a coupling capacitor.

The unbalanced-balanced conversion unit includes a transformer.

Compared with the prior art, the present invention has the following beneficial effects:

In terms of structure, the integral filter unit is connected with the unbalanced-balanced conversion unit, which can effectively suppress interference, extract partial discharge pulse signals, and interfere with each other between nonlinear loads and linear loads, and is not affected by the failure of the external power supply system of the filter and Influenced by other uncertain factors; it can be used in conjunction with other passive filters and active filters; when the frequency of interference noise such as carrier communication on site changes, there is no need to change the parameters of the filter, and the entire filter structure is simple. The operation is safe and reliable, and the installation position is flexible and diverse. the

Description of drawings

Fig. 1 is the structural principle diagram of the filter that eliminates the external noise irrelevant to partial discharge when measuring high-frequency partial discharge of the present invention;

Fig. 2 is the structural representation of integral filtering unit of the present invention;

Fig. 3 is a schematic diagram of circuit connection of a specific embodiment of the filter of the present invention.

Detailed ways

In order to facilitate a further understanding of the invention, the specific implementation manners of the invention will be further described in detail below in conjunction with the accompanying drawings.

As shown in Fig. 1, the filter that eliminates the external noise irrelevant to partial discharge during the high-frequency partial discharge measurement of the present invention comprises: input unit 100, and it has a signal input end; unit 100; and an unbalanced-balanced conversion unit 300, which is connected to the integral filter unit 200.

The input unit 100 includes an electronic switch, and the electronic switch refers to an operating unit that realizes circuit on-off, and includes at least one controllable electronic valve device.

As a specific embodiment, the passband of the filter that eliminates the external noise irrelevant to partial discharge during the high-frequency partial discharge measurement of the present invention is set to three types of options: 200 KHz to 2 MHz, 2 MHz to 20 MHz, and 15 MHz to 40 MHz.

When applying the high-frequency partial discharge measurement of the present invention to eliminate the external noise irrelevant to the partial discharge, the principle of filtering out the interference noise is: continuous periodic interference includes system high-order harmonics, high-frequency protection, carrier communication and radio communication etc. From the perspective of signal processing, various periodic interference noises can be regarded as the synthesis of harmonics, and its time-discrete signal Z(n) can be expressed as:

The spectrum of the interference noise is a discrete spectrum, and its energy is concentrated in the main frequency range, while the spectrum of the partial discharge pulse signal and the external discharge signal is a flat waveform in the entire frequency domain, and its energy is evenly distributed in the entire frequency domain.

From the perspective of spectrum analysis, the energy concentration of continuous periodic interference such as carrier communication, its amplitude spectrum is centered on the main frequency, and the pulse waveform is twice the width of the modulation frequency, while the amplitude spectrum of the internal and external discharge pulses is spread throughout the entire The flat waveform in the frequency domain, so the difference between the two amplitude spectrum values is very large, based on which the frequency of the carrier interference can be determined and removed.

For the detection signal whose number of periodic interference signals is N, n filter circuits can be cascaded, and the filter circuit system function Hi(z) of each stage is:

The corresponding difference equation is:

i为第i级滤波电路的输入，

i为第i级滤波电路的输出，

i为第i个干扰信号频率。 In the formula, a is the coefficient 0<a<1 representing the distance between the pole and the zero point, and a=0.95—0.99 is desirable,

i is the input of the i-th stage filter circuit,

i is the output of the i-th stage filter circuit,

i is the i-th interference signal frequency.

Because the partial discharge pulse has a wide frequency band, only by using a very high sampling rate can the pulse signal be retained as much as possible, and the general trend of the interference noise signal is that the higher the frequency, the lower the amplitude, so increasing the sampling frequency can also be improved to a certain extent improve the signal-to-noise ratio.

Fig. 2 is the structure schematic diagram of integral filter unit of the present invention, as shown in the figure, described integral filter unit 200 comprises the nth filter circuit, and the nth filter circuit is connected with the n+1th filter circuit, and n is greater than An integer equal to 1, in this embodiment, embodies the cascading of three filtering circuits.

Referring to Figure 1, the first filtering circuit is connected to the second filtering circuit, the second filtering circuit is connected to the third filtering circuit, the first filtering circuit is used for primary filtering, and the second filtering circuit is used for secondary filtering , the third filtering circuit is used for third filtering, which can detect the voltage distortion at the connection point between the filter and the system while filtering out the interference noise.

As a specific embodiment, the input of the electronic switching device J1 control signal of the input unit 100 passes through the integral filter unit 200, and the integral filter unit 200 includes the nth filter circuit, and the nth filter circuit is connected to the n+1th filter circuit, n is an integer greater than or equal to 1. The capacitor of the previous filter circuit performs ripple conversion on the signal, and the inductance acts as a compensation, and at the same time controls the charging voltage of the capacitor. Then, the signal enters the subsequent filter circuit for multiple filter processing.

As another embodiment of the integral filter unit shown in Figure 2, the nth filter circuit is connected to a coupling capacitor, that is, the first filter circuit is connected to a coupling capacitor, the second filter circuit is connected to a coupling capacitor, and the second filter circuit is connected to a coupling capacitor. The third filtering circuit is connected with a coupling capacitor.

In a specific embodiment, the coupling capacitor is a high-voltage capacitor connected to the power line, which is combined with the n-th filter circuit to form a carrier signal channel. The function of capacitive coupling is to transmit the AC signal from the previous stage to the next stage.

Both capacitive coupling and transformer coupling can transmit AC signals and isolate DC, so that the working points of the front and back stages are not involved with each other. But the difference is that when the capacitor is used for transmission, the phase of the signal will be delayed, and when the transformer is used for transmission, the high frequency component of the signal will be lost. In general, capacitors are commonly used as coupling elements when small signals are transmitted, and transformers are commonly used as coupling elements when large or strong signals are transmitted.

Fig. 3 is the circuit schematic diagram of the specific embodiment of filter of the present invention, as shown in the figure, described filter circuit for the first time comprises capacitor C3, and C3 is connected in series with inductance L2; Described filter circuit for the second time comprises capacitor C6, C6 is connected in series with inductor L4. The first filter circuit and the second filter circuit are composed of capacitors in series with inductors. The filter circuit generates fundamental reactive power to achieve the target power factor, which can realize the effect of reactive power compensation and filter noise removal.

The third filtering circuit includes capacitors C14, C16, C17 and inductor L6, wherein C14 is connected in series with C17, and then connected in parallel with L6, and C16 is also connected in parallel with L6. The third filter circuit is composed of capacitors in parallel with inductors, and the filter circuit can prevent overvoltage caused by unbalanced power frequency current at both ends of the filter. The first filtering circuit is used for primary filtering, the second filtering circuit is used for secondary filtering, and the third filtering circuit is used for third filtering, which can automatically detect the connection point between the filter and the system while filtering out interference noise voltage distortion.

The unbalanced-balanced conversion unit 300 includes a transformer T2, which transforms the power frequency current from unbalanced to balanced. In a specific embodiment, the primary side voltage of the transformer T2 can be upgraded from low voltage to Medium voltage or high voltage, which reduces the harmonic impedance of the system while reducing the harmonic current. Because the negative terminal of the balanced signal is separated from the ground reference, and the negative terminal of the unbalanced signal is also the ground reference terminal, using a balanced to unbalanced transformer is to isolate and couple the two signals.

In the process of signal transmission, if the signal is inverted, and then the inverted signal and the original signal are transmitted at the same time, it is called a balanced signal. If it is transmitted directly, it is an unbalanced signal. The balanced signal is sent to the differential amplifier, and the original signal and the anti-phase signal are subtracted to obtain an enhanced original signal. Since the two lines receive similar interference during transmission, the same interference is subtracted during the subtraction process. signal and therefore more immune to interference.

The transformer T2 is connected to an output filter circuit, and the output filter circuit includes: capacitors C21, C23 and C24, and the capacitors C23 and C24 are respectively connected to the capacitor C21. A transformer generally has two independent windings, primary and secondary, and these two windings share an iron core. The alternating current flows through the primary winding of the transformer to generate a magnetic potential, so there is an alternating magnetic flux in the closed iron core, the primary and secondary windings cut the magnetic field lines, and the alternating current of the same frequency can be induced in the secondary.

Due to the strong electromagnetic interference on site and the thermal noise of some analog components in the signal channel, the monitored partial discharge signal is accompanied by a large amount of noise, a large part of which can be regarded as white noise with a normal distribution. For such signals containing broadband noise, on the basis of adopting the hardware of the present invention, the wavelet transformation is performed on the software algorithm, which can be applied to the analysis of non-stationary signals.

The wavelet transform of white noise has the following properties:

Property 1: Let n(x) be real, wide and stationary white noise with variance δ2, then the expected value of wavelet transform Ws(s,x) of white noise is E(|Ws(s,x)|2)= ||ψ||2δ2/s, that is, the wavelet transform amplitude of white noise decreases with the increase of scale s.

Property 2: If the white noise n(x) is Gaussian white noise, on the scale s, the average density of its wavelet transform mode is

In the formula, ψ(1) and ψ(2) are the first and second derivatives of ψ(x), respectively.

This property shows that the average density of wavelet transform modulus of white noise decreases with the increase of scale s. In addition, it can be proved that Gaussian white noise is a distribution of consistent Lipschitz-1/2-ε, ε>0. It follows that discrete white noise is almost everywhere singular. From properties 1 and 2, it can be known that the small wavelet spectrum of white noise will gradually disappear as the scale s increases.

Wavelet transform has the localization properties required by signal characteristic analysis, especially its focusing function. The wavelet with a narrow time window reflects the high-frequency components of the signal, which can capture high-frequency transient signals and amplify its subtle features; the wavelet with a wide time window reflects the low-frequency components of the signal, that is, the trend of the signal. The reason why wavelet analysis is required is that the electrical pulse waveforms excited by the discharge pulse may be very different during internal discharge or external discharge, and there is a certain time delay, so it is difficult to distinguish directly by cross-correlation. The signal is decomposed by wavelet, the smooth component in a certain scale space has obvious phase law, and the reduction of its time domain resolution can better eliminate the time delay difference of the two discharge pulses, making it suitable for interaction Correlation operations can effectively extract the partial discharge pulse inside the transformer.

The filter of the present invention can effectively suppress interference, extract partial discharge pulse signals, and interfere with each other between nonlinear loads and linear loads, and is not affected by the failure of the external power supply system of the filter and other uncertain factors; Other passive filters are used in conjunction with active filters; when the frequency of interference noise such as carrier communication on site changes, there is no need to change the parameters of the filter. The entire filter has a simple structure, safe and reliable operation, and flexible and diverse installation positions.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention.

Claims (10)

1. a high frequency office puts and eliminates the filter of putting irrelevant external noise with office when measuring, and it is characterized in that, comprising:

Input unit, it has a signal input part;

The integral filtering unit, it connects described input unit; And

Imbalance-balance converting unit, it connects described integral filtering unit.

2. filter according to claim 1, it is characterized in that: described input unit comprises an electronic switch.

3. filter according to claim 1 and 2, it is characterized in that: described integral filtering unit comprises filter circuit the n time, and the n time filter circuit is connected with filter circuit the n+1 time, and n gets the integer more than or equal to 1.

4. filter according to claim 3, it is characterized in that: described the n time filter circuit connects a coupling capacitance.

5. filter according to claim 4 is characterized in that: described first time, filter circuit comprised capacitor C 3, the C3 inductance L 2 that is connected in series.

6. filter according to claim 5 is characterized in that: described second time, filter circuit comprised capacitor C 6, the C6 inductance L 4 that is connected in series.

7. filter according to claim 6, it is characterized in that: described filter circuit for the third time comprises capacitor C 14, C16, C17 and inductance L 6, and wherein, the C14 C17 that is connected in series is connected in parallel with L6 again, and C16 also is connected in parallel with L6.

8. filter according to claim 7, it is characterized in that: described imbalance-balance converting unit comprises transformer T2.

9. filter according to claim 8 is characterized in that: described transformer T2 connection output filter circuit.

10. filter according to claim 1, it is characterized in that: described output filter circuit comprises: capacitor C 21, C23 and C24, described capacitor C 23, C24 connect capacitor C 21 respectively.

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